A power transmission device is generally classified into a rack and pinion for converting a rotational motion into a linear motion or vice versa and a gear train for converting a rotation speed and torque while transferring only a rotational motion. Typically, a power transmission system of a power transmission device mainly uses a tooth shape based on an involute curve, but a tooth shape based on a cycloid curve and a pin gear are uncommonly used. The power transmission device is widely used for various industrial machines such as semiconductor equipment.
FIG. 1 is a side structural view schematically illustrating a use state of a conventional power transmission device 1. The conventional power transmission device 1, as illustrated in FIG. 1, may be partially coupled to a slider 3 (a hatched portion of FIG. 1) that is, for example, coupled to a base plate 2 in a structure of a rail 4 so as to allow the slider 3 to perform a linear motion.
For the linear motion of the slider 3 with respect to the base plate 2, the power transmission device 1 connected to the slider 3 are provided with a rack 5 fixed in an area of the base plate 2 and a pinion 6 engaged with the rack 5. The pinion 6 is coupled to an end portion of a shaft 7 that extends outwardly from the power transmission device 1 and engaged with the rack 5 during the coupling as shown in FIG. 1.
In the above structure, when a motor 8 of the power transmission device 1 is operated, the shaft 7 is rotated based on interaction among parts included in the power transmission device 1 and then the pinion 6 is rotated. In doing so, since the pinion 6 is engaged with the rack 5 that is fixed, the pinion 6 that rotates performs a linear motion along the lengthwise direction of the rack 5 and thus the slider 3 performs a linear motion with respect to the base plate 2. Accordingly, when a desired part or apparatus is loaded on the slider 3, the part or apparatus may be linearly moved.
The above structure of the power transmission device 1 of FIG. 1 is the most widely used structure on the spot. However, since the power transmission device 1 of FIG. 1 has a structure in which the shaft 7 protrudes outwardly and the pinion 6 is coupled to the end portion of the shaft 7, foot print, in particular, installation height H, increases so that it is difficult to apply the power transmission device 1 of FIG. 1 to compact equipment. Also, there are various problems such that complicated peripheral accessories such as a gear box or a shrink fitting to coupling are needed, the vicinity of equipment is contaminated because lubrication oil is coated on an outer surface of the pinion 6, and the maintenance work and the cleaning work are frequently needed due to its structure so that the process is delayed. Thus, improvement of a structure is needed.